For direct studies of growth control, a method was developed to purify viable human megakaryocytes to homogeneity from routine normal bone marrow aspirates. An initial separation of marrow over a 1.050 g/mL Percoll density cut was used to enrich megakaryocytes. After washing, the cells were specifically labeled with a fluoresceinated monoclonal antibody or F(ab')2 fragment to the platelet glycoprotein (GP) IIb/IIIa complex. Megakaryocytes were selectively sorted by using Becton Dickinson FACStar flow cytometer on the basis of a fluorescence intensity greater than 50-fold that of control cells. To increase resolution and purity the sorting rate was adjusted to one cell in 13 formed drops, and negative events that coincided with positive ones were aborted. Two thirds of the isolated cells were large, morphologically recognizable megakaryocytes with a forward light scatter fourfold that of the main cell population. Microscopic examination showed these cells to be greater than or equal to 98% megakaryocytes with a diameter of 20 to 46 microns and a ploidy range of 2N to 64N with a mode of 16N. The small highly fluorescent cells were 10 to 21 microns in diameter, and their ploidy range from 2N to 32N with main ploidy classes of 2N and 4N. The majority of these small cells also positively reacted with monoclonal antibody to platelet GPIb. The isolated cells were cultured in either Iscove's or leucine, lysine-deficient RPMI 1640 medium with 10% human plasma. The cells were maintained in culture more than three days and were capable of synthesis of both DNA and protein as assessed by radiolabeled thymidine and amino acid incorporation. Moreover, the isolated megakaryocytes were capable of responding to recombinant granulocyte-macrophage colony- stimulating factor. The data show that human megakaryocytes can be purified from routine marrow aspirates on the basis of a lineage marker and that they are capable of growth in vitro.